Internalization of hyaluronan by chondrocytes occurs via receptor-mediated endocytosis

Acylation of CD44 and its association with lipid rafts are required for receptor and hyaluronan endocytosis

CD44 is a cell surface receptor for the extracellular matrix macromolecule hyaluronan. In addition, CD44 mediates the endocytosis of hyaluronan leading to its subsequent degradation within lysosomes. Using model systems of COS-7 and Flp-293 cells, we demonstrate that the association of CD44 with lipid rafts is essential for the endocytosis of hyaluronan but not the extracellular binding. Further, we demonstrate that palmitoylation of CD44 on two highly conserved cysteine residues is essential for the association with lipid rafts as determined by density gradient ultracentrifugation. Mutations of either cysteine residues or pretreatment of cells with the palmitic acid analog 2-bromopalmitate, reduced the [3H]palmitic acid incorporation into CD44 and prevented CD44-lipid rafts association. Preventing CD44 palmitoylation had no effect on the binding of hyaluronan but inhibited hyaluronan internalization. The turnover of the CD44 receptor itself was also affected by blocking its association with lipid rafts. Using cycloheximide to prevent de novo protein synthesis, palmitoylation-deficient cysteine mutants underwent slower turnover from cell surface compared with the palmitoylation-intact wild type, as determined by immunofluorescence and Western blotting. These results indicate that palmitoylation of CD44 is a critical driving determinant to CD44 association with lipid rafts and, concomitantly, the rates of hyaluronan endocytosis and CD44 turnover from cell surface.

Expression and cellular localization of human hyaluronidase-2 in articular chondrocytes and cultured cell lines

OBJECTIVE

There is debate whether hyaluronan (HA) can be enzymatically degraded within the extracellular matrix of cartilage and other tissues or whether its catabolism occurs strictly within the lysosomal compartment of chondrocytes and other cell types. Previous studies have suggested that one of the lysosomal hyaluronidases (hyaluronidase-2) can be expressed as a functionally-active glycosyl phosphatidylinositol-linked protein at the surface of mammalian cells. If this form of hyaluronidase expression occurs in chondrocytes, this could represent a possible mechanism for extracellular HA cleavage. Thus, which hyaluronidases are expressed and where was the objective of this study.

METHODS

mRNA for hyaluronidases was quantified by reverse transcription-polymerase chain reaction (RT-PCR) and enzymatic activity by HA zymograms. Recombinant forms of hyaluronidase-2 were generated and expressed in model cell lines. A peptide-specific polyclonal antiserum was prepared to localize endogenous human hyaluronidase-2 in human articular chondrocytes.

RESULTS

Hyaluronidase-2 is the principal mRNA transcript expressed by primary human articular chondrocytes as well as various model cell lines. Recombinant hyaluronidase-2, containing N-terminal or C-terminal epitope tags, was strictly localized intracellularly and not released by treatment with a phosphatidylinositol-specific phospholipase. Endogenous hyaluronidase-2 expressed by human chondrocytes as well as HeLa cells could only be detected following detergent permeabilization of the plasma membranes.

CONCLUSIONS

These data suggest that on chondrocytes and other cell types examined, hyaluronidase-2 is not present or functional at the external plasma membrane. Thus, local turnover of HA is dependent on receptor-mediated endocytosis and delivery to low pH intracellular organelles for its complete degradation.

Effects of vibration and hyaluronic acid on activation of three-dimensional cultured chondrocytes

OBJECTIVE

To investigate the effects of vibration (Vib) and hyaluronic acid (HA) on 3-dimensional cultured cartilage.

METHODS

Chondrocytes were obtained from metatarsophalangeal joints of freshly killed 6-month-old pigs. Twenty-four-well plates containing type I collagen sponge disks were used to culture samples. The frequency and the amplitude of the vibration of the well plate were 100 Hz and 0.5 nm, respectively. We produced 3-dimensional cartilage tissue using HA and vibration with collagen sponge as a carrier. Four different culture conditions were examined: a control HA-Vib- group, an HA-Vib+ group, an HA+Vib- group, and an HA+Vib+ group. Each group was cultured for 2 weeks. After culture days 3, 7, 10, and 14 (every 3.5 days), the levels of chondroitin 4-sulfate (C4S) and chondroitin 6-sulfate (C6S) isomers synthesized in each culture medium were measured. Histologic analysis, immunohistochemical analysis, and electron microscopic examination were performed.

RESULTS

Mean C4S and C6S synthesis had increased rapidly after 7 days of culture and continued to increase thereafter. There were significant differences among the 4 groups (P < 0.01). Synthesis of both C4S and C6S was most abundant in the HA+Vib+ group and the lowest in the HA-Vib- group. After 1 and 2 weeks of culture, the chondrocytes had formed stratified structures on the collagen sponges in all groups, although the thickest structure was observed in the HA+Vib+ group and the thinnest in the HA-Vib- group. Under immunofluorescence, the HA+Vib+ group exhibited the strongest chromatic features. Under electron microscopy, the chondrocytes in the HA+Vib+ group exhibited many long and slender prominences on their surface, and extracellular substance could be observed associated with the cells.

CONCLUSION

Our results indicate that the combination of vibration and HA activates the production of proteoglycan in 3-dimensional cultured chondrocytes and stimulates MAPK and beta-catenin. This suggests that some mechanoreceptors for vibration exist on the plasma membrane of chondrocytes and activate the intracellular signal transduction system.

Overexpression of hyaluronan synthase 2 alters hyaluronan distribution and function in proximal tubular epithelial cells

The functional consequences of increased renal cortical hyaluronan that is associated with both acute injury and progressive scarring are unclear. The aim of this study was to characterize hyaluronan synthase-2 (HAS2)-driven HA synthesis and determine its effect on renal proximal tubular epithelial cell (PTC) function, because this is known to be the inducible form of HA synthase in this cell type. Overexpression of HAS2 mRNA increased HA generation, which in the supernatant predominantly was HA of large molecular weight, whereas there was an increase in low molecular weight HA in cell-associated fractions. This was associated with increased expression of hyaluronidases, inhibition of HA cable formation concurrent with reduction in HA-dependent monocyte binding, and increased pericellular HA matrix. Overexpression of HAS2 led to enhanced cell migration. HA can be modified by the covalent attachment of heavy chains that are derived from the serum protein inter-alpha-inhibitor (IalphaI), a process that is known to be catalyzed by TNF-alpha-stimulated gene 6 (TSG-6; an inflammation-associated protein). Enhanced migration was abrogated by blocking antibodies to either IalphaI or TSG-6. Addition of recombinant full-length TSG-6 (TSG-6Q) or TSG-6Q_Y94F, a mutant variant with impaired HA binding, increased cell migration. Both of these proteins were able to mediate the covalent transfer of heavy chains, from IalphaI and pre-alpha-inhibitor, onto HA. Addition of the isolated TSG-6-Link module (Link_TSG-6), which binds HA but is unable to form covalent complexes with IalphaI/pre-alpha-inhibitor, had no effect on migration, suggesting that TSG-6-mediated formation of heavy chain-HA complexes is critical in the formation of a pericellular HA matrix.

The accumulation of intracellular ITEGE and DIPEN neoepitopes in bovine articular chondrocytes is mediated by CD44 internalization of hyaluronan

OBJECTIVE

A dramatic loss of aggrecan proteoglycan from cartilage is associated with osteoarthritis. The fate of residual G1 domains of aggrecan is unknown, but inefficient turnover of these domains may impede subsequent repair and retention of newly synthesized aggrecan. Thus, the objective of this study was to determine whether ITEGE- and DIPEN-containing G1 domains, generated in situ, are internalized by articular chondrocytes, and whether these events are dependent on hyaluronan (HA) and its receptor, CD44.

METHODS

ITEGE and DIPEN neoepitopes were detected by immunofluorescence staining of bovine articular cartilage chondrocytes treated with or without interleukin-1alpha (IL-1alpha). Additionally, purified ITEGE- or DIPEN-containing G1 domains were aggregated with HA and then added to articular chondrocytes, articular chondrocytes transfected with CD44delta67, or COS-7 cells transfected with or without full-length CD44. Internalized epitopes were distinguished by their resistance to extensive trypsinization of the cell surface.

RESULTS

Both ITEGE and DIPEN were visualized within the extracellular cell-associated matrix of chondrocytes as well as within intracellular vesicles. Following trypsinization, the intracellular accumulation of both epitopes was clearly visible. IL-1 treatment increased extracellular as well as intracellular ITEGE epitope accumulation. Once internalized, the ITEGE neoepitope became localized within the nucleus and displayed little colocalization with HA, DIPEN, or other G1 domain epitopes. The internalization of both ITEGE and DIPEN G1 domains was dependent on the presence of HA and CD44.

CONCLUSION

One important mechanism for the elimination of residual G1 domains following extracellular degradation of aggrecan is CD44-mediated co-internalization with HA.

Recent advances in tumor-targeting anticancer drug conjugates

Traditional cancer chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be a killed by cytotoxic agent. In reality, however, cytotoxic agents have very little or no specificity, which leads to systemic toxicity, causing severe undesirable side effects. Therefore, various drug delivery protocols and systems have been explored in the last three decades. Tumor cells overexpress many receptors and biomarkers, which can be used as targets to deliver cytotoxic agents into tumors. In general, a tumor-targeting drug delivery system consists of a tumor recognition moiety and a cytotoxic warhead connected directly or through a suitable linker to form a conjugate. The conjugate, which can be regarded as 'prodrug', should be systemically non-toxic. This means that the linker must be stable in circulation. Upon internalization into the cancer cell the conjugate should be readily cleaved to regenerate the active cytotoxic agent. Tumor-targeting conjugates bearing cytotoxic agents can be classified into several groups based on the type of cancer recognition moieties. This review describes recent advances in tumor-targeting drug conjugates including monoclonal antibodies, polyunsaturated fatty acids, folic acid, hyaluronic acid, and oligopeptides as tumor-targeting moieties.

The interaction of versican with its binding partners

Versican belongs to the family of the large aggregating chondroitin sulfate proteoglycans located primarily within the extracellular matrix (ECM). Versican, like other members of its family, has unique N- and C-terminal globular regions, each with multiple motifs. A large glycosaminoglycan-binding region lies between them. This review will begin by outlining these structures, in the context of ECM proteoglycans. The diverse binding partners afforded to versican by virtue of its modular design will then be examined. These include ECM components, such as hyaluronan, type I collagen, tenascin-R, fibulin-1, and -2, fibrillin-1, fibronectin, P- and L-selectins, and chemokines. Versican also binds to the cell surface proteins CD44, integrin beta 1, epidermal growth factor receptor, and P-selectin glycoprotein ligand-1. These multiple interactors play important roles in cell behaviour, and the roles of versican in modulating such processes are discussed.

Hyaluronan fragments activate nitric oxide synthase and the production of nitric oxide by articular chondrocytes

Chondrocyte CD44 receptors anchor hyaluronan to the cell surface, enabling the assembly and retention of proteoglycan aggregates in the pericellular matrix. Hyaluronan-CD44 interactions also provide signaling important for maintaining cartilage homeostasis. Disruption of chondrocyte-hyaluronan contact alters CD44 occupancy, initiating alternative signaling cascades. Treatment with hyaluronan oligosaccharides is one approach to uncouple CD44 receptors from its native ligand, hyaluronan. In bovine articular chondrocytes, treatment with hyaluronan oligosaccharides or purified hyaluronan hexasaccharides induced the production of nitric oxide that mirrored nitric oxide production following interleukin-1 treatment. In contrast, 120 and 1,260 kDa hyaluronan did not induce production of nitric oxide. Human chondrocytes responded similarly to treatment with hyaluronan or hyaluronan oligosaccharides. Nitric oxide production from chondrocytes was mediated by activation of the inducible nitric oxide synthase, as confirmed by mRNA expression and inhibition of nitric oxide production by diphenyleneiodonium. Co-treatment of chondrocytes with hyaluronan oligosaccharides and interleukin-1 did not demonstrate additive effects. Blocking interleukin-1 receptors with an antagonist did not abolish the production of nitric oxide induced by treatment with hyaluronan oligosaccharides. Moreover, only COS-7 following transfection with a pCD44, not the CD44-null parental cells, responded to treatment with hyaluronan oligosaccharides by releasing nitric oxide. This study demonstrates a novel signaling potential by hyaluronan fragments, in lieu of endogenous hyaluronan-chondrocyte interactions, resulting in the activation of inducible nitric oxide synthase.

Evaluation of hyaluronidase activity in equine and bovine sera and equine synovial fluid samples by use of enzyme zymography

OBJECTIVE

To investigate the activities of hyaluronidases in equine sera and synovial fluid samples and sera from fetal and adult bovids and evaluate the extent to which the degradation of hyaluronan is influenced by chondrocytes.

SAMPLE POPULATION

Commercial and noncommercial samples of equine (n = 6) and bovine (6) sera and 16 synovial fluid samples from horses.

PROCEDURE

Hyaluronidase activities in sera and synovial fluid samples were assessed via enzyme zymography (performed at pH 4, 5, 6, or 7). Chondrocytes were isolated from equine cartilage and cultured with or without hyaluronan (1 mg/mL); the degradation of hyaluronan was assessed via agarose gel electrophoresis.

RESULTS

[corrected] Hyaluronidase activity was detected in equine sera and synovial fluid samples at pH 4, but not at pH 7, and in bovine sera at both pH values. In all samples at pH 4, a major band of activity (molecular weight, approx 60 kd) and some additional higher molecular weight bands were detected; high- and low-molecular-weight activities were detected in bovine sera at pH 7 Hyaluronan in tissue culture medium with or without fetal calf serum was degraded in the presence, but not the absence, of equine chondrocytes.

CONCLUSIONS AND CLINICAL RELEVANCE

Hyaluronidase activity was detected in equine sera and synovial fluid at pH 4 and in bovine sera at pH 4 and 7. Primary chondrocytes in monolayer culture can degrade exogenous hyaluronan. Modulating native hyaluronidase activity may offer a new approach to improve the quantity and quality of hyaluronan in articular joints.

Characterization of promoter elements of the human HYAL-2 gene

The regulated catabolism of hyaluronan is critical to the function of many connective tissues. In cartilage, hyaluronan catabolism occurs locally by resident chondrocytes. To determine whether the expression of lysosomal hyaluronidases contributes to this regulation, the promoter elements associated with HYAL-2 gene expression were characterized. Human articular chondrocytes were found to express all three lysosomal hyaluronidases, HYAL-1, HYAL-2, and HYAL-3. HYAL-2 was the predominant gene product. Using 5' RACE (rapid amplification of cDNA ends) analysis, multiple transcription initiation sites were identified including a novel initiation site located within intron 1 of the gene expressed by human articular chondrocytes. The presence of multiple transcriptional initiation sites is a typical feature of TATA-less promoter regions, such as those of HYAL-2. Approximately 4000 bp of 5' flanking sequence of the HYAL-2 gene was characterized. Transient transfection of C-28/I2 cells with various 5' deletion constructs indicated that the region between +959 to +1158 (within intron 1) contains the basal promoter for HYAL-2 in chondrocytes. In addition, the region +224 to +958 contained a negative modulator that could control the basal expression level of HYAL-2. Treatment of human articular chondrocytes or C-28/I2 cells with various catabolic cytokines did not alter HYAL-2 mRNA expression, luciferase promoter expression, or hyaluronidase enzymatic activity. Thus, in chondrocytes HYAL-2 appears to be constitutively expressed and not inducibly regulated by catabolic agents. As such, it appears that the expression of lysosomal hyaluronidase participates little in the overall regulation of hyaluronan catabolism.

The metabolism of hyaluronan in cultured rabbit growth plate chondrocytes during differentiation

Hyaluronan (HA) is one of the major extracellular matrix components in cartilage. In addition to the biomechanical functions, HA has various important roles in the differentiation of chondrocytes. The purpose of this study was to clarify the nature of HA synthesis during chondrocyte differentiation. Growth plate chondrocytes were isolated from rabbit ribs and cultured in chondrocyte differentiation medium. The amount of HA and HA synthase (HAS) mRNA levels were analyzed for each stage of chondrocyte differentiation by means of high-performance liquid chromatography (HPLC) and real-time PCR, respectively. The distribution of HA in cultured chondrocytes was observed by histochemical staining. The amount of HA, ranging widely in size, was increased substantially during the hypertrophic stage. The expression levels of HAS2 and HAS3 mRNAs were low during the matrix-forming stage. HAS2 mRNA level was substantially enhanced at the pre-hypertrophic stage, whereas HAS3 mRNA level exhibited a slight increase. HAS1 mRNA was not detected. The intensity of HA staining was high around the hypertrophic chondrocytes. These results suggest that HA metabolism in chondrocyte differentiation is regulated by the selective expression of HASs, and HAS2 and the related large size-HA may have a certain association with the hypertrophic changes of chondrocytes.

A new role for chondrocytes as non-professional phagocytes. An in vitro study

Chondrocytes are capable of engulfing latex particles, cell detritus, and necrotic and apoptotic remains in vitro. It is conceivable that chondrocytes might be involved in the clearance by phagocytosis of different materials within the cartilage. In fact, so far there is no evidence for the presence of "professional phagocytes" (macrophages and neutrophils) in this tissue. Chondrocyte suspensions obtained from rat knees and hips were cultured to assess phagocytosis of latex particles (1 microm), articular cartilage detritus, and necrotic and apoptotic chondrocyte remains (induced by VP-16 1 mM). We observed that chondrocytes phagocytosed latex particles as evaluated by confocal microscopy and flow cytometry. In addition, we observed that chondrocytes phagocytosed articular cartilage detritus and necrotic and apoptotic VP-16 induced-chondrocytes, as observed by bright field microscopy and transmission electron microscopy.

Hyaluronan catabolism: a new metabolic pathway

A new pathway of intermediary metabolism is described involving the catabolism of hyaluronan. The cell surface hyaluronan receptor, CD44, two hyaluronidases, Hyal-1 and Hyal-2, and two lysosomal enzymes, beta-glucuronidase and beta-N-acetylglucosaminidase, are involved. This metabolic cascade begins in lipid raft invaginations at the cell membrane surface. Degradation of the high-molecular-weight extracellular hyaluronan occurs in a series of discreet steps generating hyaluronan chains of decreasing sizes. The biological functions of the oligomers at each quantum step differ widely, from the space-filling, hydrating, anti-angiogenic, immunosuppressive 10(4)-kDa extracellular polymer, to 20-kDa intermediate polymers that are highly angiogenic, immuno-stimulatory, and inflammatory. This is followed by degradation to small oligomers that can induce heat shock proteins and that are anti-apoptotic. The single sugar products, glucuronic acid and a glucosamine derivative are released from lysosomes to the cytoplasm, where they become available for other metabolic cycles. There are 15 g of hyaluronan in the 70-kg individual, of which 5 g are cycled daily through this pathway. Some of the steps in this catabolic cascade can be commandeered by cancer cells in the process of growth, invasion, and metastatic spread.

Treatment with calcitonin prevents the net loss of collagen, hyaluronan and proteoglycan aggregates from cartilage in the early stages of canine experimental osteoarthritis

OBJECTIVE

To evaluate the effect of calcitonin (CT) on the histology and biochemistry of articular cartilage from unstable operated and nonoperated knee in a canine model of experimental osteoarthritis (OA).

METHODS

Eighteen dogs underwent anterior cruciate ligament transection (ACLT) of the right knee and were randomly distributed into three groups of six dogs each. From day-1 after surgery until sacrifice 84 days post-ACLT, each dog received a daily nasal spray that delivered the placebo, 100 units of CT or 400 units of CT. Histologic lesions were scored. Hyaluronan (HA) and antigenic keratan sulfate (AgKS) were quantified by enzyme-linked immunosorbent assays (ELISAs), whereas aggrecan molecules extracted under nondissociative conditions were characterized by velocity gradient centrifugation.

RESULTS

All canine cruciate-deficient knees developed OA. At a daily dose of 400 units, CT had no effect on the size of osteophytes but significantly reduced the severity of cartilage histologic lesions in unstable knees. CT also enhanced the HA content as well as the size distribution and relative abundance of fast-sedimenting aggrecan aggregates in cartilage from both operated and nonoperated knees. On the other hand, in the CT-treated group, the cartilage content of AgKS increased in operated joints, but not in nonoperated joints.

CONCLUSIONS

Because CT delivered as a nasal spray markedly reduced the severity of most OA changes, both at the histological and biochemical level, this form of therapy may have benefits for humans who have recently experienced a traumatic knee injury, and as well as for dogs who spontaneously rupture their ACL.

Hyaluronidase expression in cultured growth plate chondrocytes during differentiation

Hyaluronan (HA) is a major component of the extracellular matrix of cartilage, contributes to its structural and functional integrity, and has various important roles in the differentiation of chondrocytes. HA metabolism is regulated by both anabolic and catabolic processes; however, the details have not yet been clarified. The purpose of this study was to clarify the expression patterns of hyaluronidase (HAase) mRNAs (from the relevant HAase genes: the HYALs) and HAase activity during chondrocyte differentiation. Cartilage tissue and growth plate chondrocytes were isolated from the ribs of 4-week-old male Japanese rabbits. The expression of HYAL mRNAs in cartilage was analyzed by in situ hybridization. The expression levels of HYAL mRNAs in the culture were analyzed for each of the chondrocyte differentiation stages by means of quantitative real-time polymerase chain reaction analysis. Enzymatic activity in the conditioned medium from the cultures was examined by using HA zymography and an enzyme-linked immunosorbent-like assay. The expression levels of HYAL1 and HYAL2 mRNAs were enhanced about 2.8-fold and 3.2-fold at the maximum during the early matrix forming stage, respectively, and by about 3.2-fold and 2.0-fold at the maximum in the hypertrophic stage, respectively. HYAL3 mRNA was not detected throughout the experimental period. HAase activity was enhanced at the early matrix forming and hypertrophic stages. These results suggest that selective expression of HYALs is essential for extracellular HA metabolism during chondrocyte differentiation.

Expression of the CD44 variant isoform 5 in the human osteoarthritic knee joint: correlation with radiological, histomorphological, and biochemical parameters

PURPOSE

The purpose of this study was to correlate expression of CD44v5 in osteoarthritic synovium, cartilage, and synovial fluid with radiographical, histomorphological, and biochemical data.

METHODS

Cartilage and synovia specimens of 27 patients with osteoarthritis were histomorphologically assessed according to Mankin and Pelletier, respectively. Extended weight-bearing antero-posterior radiographs were evaluated according to Kellgren and Ahlback. Expression of membrane-bound CD44v5 was analyzed by immunohistochemistry and levels of soluble CD44v5 were determined by ELISA.

RESULTS

Expression of CD44v5 in cartilage and synovia was detected in 67% and 59% of the patients, respectively. Immunohistochemical findings in cartilage correlated significantly with structural cartilage changes (p < 0.001), whereas no correlation was found between expression in synovia and inflammatory synovial changes. Additionally, no relationship was evident between CD44v5 expression and radiographical data, but expression in cartilage and synovium was significantly correlated with each other (p < 0.04). Surprisingly, expression of CD44v5 in both cartilage and synovia was negatively correlated with synovial fluid levels of TNFalpha (p < 0.03 and p < 0.02, respectively), and no association was evident with levels of IL-1beta.

CONCLUSIONS

The data demonstrate expression of CD44v5 in osteoarthritic cartilage and synovia, probably independent of joint inflammation. But more importantly, expression of this receptor variant in cartilage seems to be strongly related to the degree of cartilage destruction.

Recruitment of IFN-gamma-producing (Th1-like) cells into the inflamed retina in vivo is preferentially regulated by P-selectin glycoprotein ligand 1:P/E-selectin interactions

Although there is evidence that altering the Th1/Th2 balance toward Th2 cells may be important in the resolution of Th1-type autoimmune disease, adoptive transfer of Th2 cells is not effective in protecting against Th1-type disease and may cause disease. Therefore, we examined the recruitment of Th1- and Th2-like cells into the retina in the murine autoimmune disease experimental autoimmune uveoretinitis. CD4 T cells were polarized in vitro to IFN-gamma-producing Th1-like cells and non-IFN-gamma-producing Th2-like cells, labeled, and adoptively transferred. Trafficking to the retina in vivo was evaluated by scanning laser ophthalmoscopy and infiltration by confocal microscopy. There were more rolling and adherent Th1-like cells and they rolled more slowly than did Th2-like cells. Th1-like cells were preferentially recruited into the retinal parenchyma at both initiation and resolution. Surface P-selectin glycoprotein ligand 1 (PSGL-1) and LFA-1 were up-regulated on both populations but were expressed at higher levels on Th1-like cells. Up-regulation of CD44 expression was higher on Th2-like cells. P-selectin, E-selectin, and ICAM-1 are up-regulated on postcapillary venules in the retina. Pretreatment of Th1-like cells with anti-PSGL-1 inhibited rolling and infiltration of Th1-like cells but not Th2-like cells, providing direct in vivo evidence for the inability of Th2 to respond to P/E-selectin despite increased expression of PSGL-1. Anti-LFA-1 pretreatment inhibited infiltration of both Th1- and Th2-like cells, but more so Th-1. We suggest that random trafficking of activated T cells (both Th1 and Th2) across the blood-retina barrier is mediated by CD44:CD44R and LFA-1:ICAM-1, whereas preferential recruitment of Th1 cells is mediated by PSGL-1:P/E-selectin.

G1 domain of aggrecan cointernalizes with hyaluronan via a CD44-mediated mechanism in bovine articular chondrocytes

OBJECTIVE

To determine whether aggrecan fragments bound to hyaluronan (HA) can be retained and internalized by articular chondrocytes and whether these events are dependent on HA and its receptor, CD44. An additional objective was to determine whether partial degradation of aggrecan is a prerequisite for internalization.

METHODS

Binding and internalization of a variety of fluorescein isothiocyanate (FITC)- or biotin-labeled HA/proteoglycan probes were investigated on normal bovine articular cartilage chondrocytes, bovine articular chondrocytes transfected with a dominant-negative construct of CD44, or COS-7 cells transfected with wild-type CD44. The probes were defined as being internalized by the presence of label associated with the cells following extensive trypsinization of the cell surface.

RESULTS

Biotinylated aggrecan fragments bound to FITC-HA were cointernalized in bovine articular chondrocytes or COS-7 cells transfected with CD44. Intracellular vesicles containing FITC-HA colocalized with a fluorescent probe for lysosomes. The internalization of the aggrecan fragments was dependent on the presence of HA as well as the presence of functional CD44. Intact aggrecan/FITC-HA complexes bound to the cell surface but were not internalized. However, following brief trypsin digestion of the aggrecan/HA complex, the remaining proteoglycan fragments were bound and internalized.

CONCLUSION

Partially degraded aggrecan fragments (e.g., aggrecan G1 domains bound to HA) can be internalized by articular chondrocytes via a mechanism involving HA/CD44-mediated endocytosis. Further, the presence of an intact aggrecan monomer bound to HA inhibits the internalization of HA as well as HA-bound fragments.

The role of renal proximal tubular cells in diabetic nephropathy

We now know that the rate of progression of diabetic nephropathy, like all progressive renal disease, correlates with the degree of corticointerstitial fibrosis. Therefore, much interest has focused on the contribution of the resident cells in the renal cortex to this process. This article reviews the evidence that the epithelial cells of the proximal tubule are major players in orchestrating events in the corticointerstitium in diabetic nephropathy. More specifically, it addresses their role in extracellular matrix turnover, generation of cytokines, and recruitment of inflammatory cells, as well as examining the concept that they are the source of the interstitial myofibroblasts, which are the principal mediators of the fibrotic process.

CD44H and the isoforms CD44v5 and CD44v6 in the synovial fluid of the osteoarthritic human knee joint

OBJECTIVE

To determine whether the concentrations of CD44H and v5 and v6 in the synovial fluid are correlated with the presence of accompanying synovitis in the osteoarthritic joint and with the grade of osteoarthritis.

DESIGN

Using antero-posterior and lateral X-rays of the knee joint and patellar view of 46 patients were graded with the Kellgren & Lawrence scale. Synovial fluid from these patients with different grades of primary osteoarthritis of the knee joint with and without synovial inflammation (synovitis) was collected during surgical procedures. A horseradish peroxidase conjugated anti-CD44H-, anti-sCD44v5- or anti-sCD44v6-antibody was added and labeled with tetramethylbenzidine. The absorbance was measured at wavelengths of 450/620 nm. Regression analysis was performed and the statistical significance was assessed, using the Student t-test for unequal variance.

RESULTS

CD44H and v5 and v6 were detected in the synovial fluid of all 46 patients. Osteoarthritic patients with synovial inflammation showed significantly higher levels of CD44H and v6, but not v5, than osteoarthritic patients without synovial inflammation. With progression of osteoarthritis from Kellgren grade II to III, CD44v5 decreased significantly. All other isoform concentrations remained statistically unchanged.

CONCLUSIONS

CD44H and the isoforms v5 and v6 were present in the synovial fluid of osteoarthritic patients. Their concentrations do not reflect the osteoarthritic stage in the Kellgren grading scale. CD44H and CD44v6, but not CD44v5, are significantly up-regulated in osteoarthritic synovial inflammation.

High binding capacity of cyclophilin B to chondrocyte heparan sulfate proteoglycans and its release from the cell surface by matrix metalloproteinases: possible role as a proinflammatory mediator in arthritis

OBJECTIVE

To study cyclophilin B, a protein newly identified as a secretion product of cultured chondrocytes, in the context of chondrocyte pathobiology.

METHODS

Cyclophilin B was purified by sequential chromatographic processing of the secretion medium of cultured guinea pig chondrocytes. Its presence both at the surface of chondrocyte monolayers and in cartilage was demonstrated by immunohistochemistry. Binding sites at the surface of chondrocytes were characterized by Scatchard plot analysis using (125)I-labeled cyclophilin B, and by glycosidase treatments. The release of cyclophilin B from chondrocytes by activated matrix metalloproteinases (MMPs) was studied by Western blot analysis.

RESULTS

Cyclophilin B was present at the surface of cultured chondrocytes and within cartilage, both in cells and in the extracellular matrix, with a particularly intense staining in the superficial layer. It was secreted constitutively by chondrocytes and cartilage explants. Its secretion was enhanced after treatment with its pharmacologic binding partner, cyclosporin A (CSA). Experiments with (125)I-labeled cyclophilin B demonstrated the presence of high-capacity, low-affinity, NaCl-sensitive binding sites at the surface of chondrocytes. Cell-bound cyclophilin B could be released by heparinase treatment, demonstrating binding to pericellular heparan sulfate proteoglycans (HSPGs). Chondroitinase or keratanase treatments had no effect. MMPs 1, 2, 3, 9, and 13 released intact cyclophilin B from the cell surface, probably by cleavage of HSPGs. This effect was reversed by the broad-spectrum MMP inhibitor, marimastat.

CONCLUSION

Cyclophilin B is a secreted CSA-binding protein involved in inflammatory events. It can induce chemotaxis in human neutrophils and T lymphocytes. The finding that cyclophilin B is an intrinsic component of cartilage and that it can be released by MMPs suggests that it has a role in the pathogenesis of arthritic diseases, even more so since its signaling receptor is present within the inflamed joint both on T cells and in the rheumatoid synovium.

Hyaluronan and CD44: strategic players for cell-matrix interactions during chondrogenesis and matrix assembly

Embryonic induction, soluble and insoluble factors, receptors, and signal transduction are orchestrated for the morphogenesis of the cartilage elements. The interaction of cells with the extracellular matrix (ECM) may lead to altered cellular response to morphogens based on the formation of new adhesive contacts, or the uncoupling of cell-matrix interactions. Hyaluronan's influence on cell behavior, and its intimate association with cells are accomplished by a wide variety of specific binding proteins for hyaluronan. The temporal expression of the hyaluronan receptor CD44 (which is expressed as several alternatively spliced variants) may be strategic to many of these cell-matrix interactions during chondrogenesis. CD44 expression is temporally coincident with the reduction of intercellular spaces at the regions of future cartilage deposition. The spatial organization of CD44 at the cell surface may function to establish or regulate the structure of the pericellular matrix dependent on a hyaluronan scaffold. As the ECM is modified during embryogenesis, the cellular response to inductive signals may be altered. An uncoupling of chondrocyte-hyaluronan interaction leads to chondrocytic chondrolysis. Thus, consideration of cell-matrix interactions during chondrogenesis, in the light of our current understanding of the temporal and spatial expression of signaling morphogens, should become a promising focus of future research endeavors.

The effect of hyaluronan on CD44-mediated survival of normal and hydroxyl radical-damaged chondrocytes

OBJECTIVE

To identify the CD44-receptor-mediated effects of 5-7 x 10(5)MW hyaluronan (HA, Hyalgan) on cell viability in normal and damaged human chondrocyte primary cultures isolated from articular cartilage.

DESIGN

Primary cultures of human chondrocytes were established from normal articular biopsies and expanded to the second culture passage. The dose-response effects of HA on the viability of normal cultures were identified. Chondrocytes were then treated with either hypoxanthine (2 mM) and xanthine oxidase (20-60 mU), or with activated polymorphonuclear leukocytes (PMNs) to induce injury. Damaged and control cells were then treated with 5-7 x 10(5)HA in the previously identified optimal dose of 0.05 mg/ml. Viability was assessed at specific time periods for the chemically and PMN-damaged cells. To identify if HA effects were mediated by the CD44 receptor, chondrocytes were incubated with anti-CD44 antibody at saturating concentrations (5 microg/ml for 100,000 cells) to produce a maximum inhibition of HA binding. Cells were evaluated using the MTT viability assay, histology, electron microscopy and immunohistochemistry.

RESULTS

Direct addition of HA (optimal dose, 0.5 mg/ml) significantly increased cell survival in normal chondrocyte primary cultures (P<0.05). Similarly, addition of this same dose of HA to cultures of free radical-damaged chondrocytes, restored the viability to baseline conditions. Cell viability rates dropped significantly (P<0.05) when CD44 receptor binding was inhibited, indicating that cell growth was mediated by the CD44 receptor.

CONCLUSIONS

HA (0.5 mg/ml of 5-7 x 10(5)) significantly increased the viability of normal human chondrocytes in primary culture and restored cell viability to near normal levels after oxidative cell injury.

Expression and activity of hyaluronidase in human periodontal ligament fibroblasts

BACKGROUND

Hyaluronan is a major component of the extracellular matrix of periodontal ligament (PDL) contributing to the structural and functional integrity. Hyaluronans contribute to the buffering effect of the PDL during chewing, and they are also important in inflammation and wound healing. Hyaluronan is known to be synthesized and turned over by the resident PDL cells, although the mechanisms of hyaluronan metabolism still remain unclear. Hyaluronidase (HAase), an endoglycosidase, degrades hyaluronan into small fragments. Currently, 3 human HAases, HYAL1, HYAL2, and PH-20, have been identified and well characterized.

METHODS

This study was conducted to investigate the expression and activity of these HAases in cultured human PDL fibroblasts and to elucidate the mechanisms involved in hyaluronan metabolism under normal and inflammatory conditions. Human PDL fibroblasts derived from the periodontium of 3 premolars were cultured with or without interleukin (IL)-1beta (0.1 to 10 ng/ml) and tumor necrosis factor (TNF)-alpha (1 to 100 ng/ml) for 0 to 48 hours. The expression of HAase mRNA was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time PCR, and the enzymatic activity was examined using hyaluronan zymography.

RESULTS

PDL fibroblasts expressed HYAL1 and HYAL2 mRNAs, but not PH-20 mRNA. The expression of HYAL1 mRNA was enhanced by about 3.5- and 3.7-fold at maximum after 1-hour stimulation with 1 ng/ml IL-1beta and after 3-hour stimulation with 10 ng/ml TNF-alpha, respectively. The expression of HYAL2 and PH-20 mRNAs was not affected by stimulation with cytokines. HAase activity was detected in conditioned medium from PDL fibroblast cultures, and the activity was enhanced by treatment with 10 ng/ml TNF-alpha.

CONCLUSION

These results suggest that PDL fibroblasts express HAases and generate HAase activity essential for extracellular hyaluronan metabolism under physiological and inflammatory conditions.

Latrunculin and cytochalasin decrease chondrocyte matrix retention

The proteoglycan-rich extracellular matrix (ECM) directly associated with the cells of articular cartilage is anchored to the chondrocyte plasma membrane via interaction with the hyaluronan receptor CD44. The cytoplasmic tail of CD44 interacts with the cortical cytoskeleton. The objective of this study was to determine the role of the actin cytoskeleton in CD44-mediated matrix assembly by chondrocytes and cartilage matrix retention and homeostasis. Adult bovine articular cartilage tissue slices and isolated chondrocytes were treated with latrunculin or cytochalasin. Tissues were processed for histology and chondrocytes were examined for CD44 expression and pericellular matrix assembly. Treatments that disrupt the actin cytoskeleton reduced chondrocyte pericellular matrix assembly and the retention of proteoglycan within cartilage explants. There was enhanced detection of a neoepitope resulting from proteolysis of aggrecan. Cytoskeletal disruption did not reduce CD44 expression, as monitored by flow cytometry, but detergent extraction of CD44 was enhanced and hyaluronan binding was decreased. Thus, disruption of the cytoskeleton reduces the anchorage of CD44 in the chondrocyte membrane and the capacity of CD44 to bind its ligand. The results suggest that cytoskeletal disruption within cartilage uncouples chondrocytes from the matrix, resulting in altered metabolism and deleterious changes in matrix structure.

Thin film rheology and lubricity of hyaluronic acid solutions at a normal physiological concentration

Using a surface forces apparatus to measure forces, and optical (multiple beam) interferometry to measure surface shapes and separations (to +/-1 A), the normal, viscous, and shear (lubrication) forces between smooth mica surfaces in aqueous hyaluronic acid (HA) solutions were measured. The experimental conditions of loading pressures, pH, and HA concentration were set to closely correspond to physiological human knee-joint conditions. From the force and optical (refractive index) measurements, it was concluded that, like other negatively charged polyelectrolytes, HA does not naturally adsorb on the mica surface which is hydrophilic and weakly negatively charged at physiological conditions: the polymer solution exhibits the bulk viscosity (22.5 +/- 1.5 cP) for films thicker than about 0.4 miccrom of the polymer, whereas for thinner films, the viscosity decreases monotonically toward the value of the pure electrolyte solution (1 cP) as HA is extruded from between the surfaces. This is indicative of a repulsive "depletion" interaction of HA with each mica surface and to a weakly attractive polymer-mediated force between the two surfaces. Thus, free HA in synovial fluid is not expected to act as a good "boundary lubricant." Relaxation measurements on approaching and receding surfaces in HA solutions were also performed, and it is shown that the presence of HA in the bulk solution can improve "hydrodynamic" modes of lubrication, for example, by assuaging the compression stroke. The study includes information that is beneficial to researchers working with biomaterials viscosupplementation devices.

Preliminary observations on the influence of rheumatoid alpha-1-acid glycoprotein on collagen fibril formation

This study investigates the effect of alpha(1)-acid glycoprotein (AGP) isolated from both normal and rheumatoid plasma on type II collagen fibril formation. Rheumatoid samples were obtained over 2 years from two patients with early arthritis. The glycosylation of each sample was analysed to establish any correlation with fibrillogenesis. Rheumatoid AGP displays increased fucosylation compared to normal AGP. In both patients the fucosylation dipped after 1 year, then rose again over year 2. It is proposed that year 1 corresponds to the acute phase of the disease and the onset of chronic inflammation after this time produces a subsequent increase in fucosylation. Rheumatoid AGP influences type II collagen fibrillogenesis. Native fibrils were produced but with differences in the rate and extent of fibrillogenesis depending on AGP concentration and fucosylation. Low concentrations produced a decrease in fibrillogenesis rate and fibril diameter. High concentrations produced fibrils at a rate and diameter dependent on fucosylation. Highly fucosylated AGP produced narrow fibrils slowly, whereas poorly fucosylated AGP produced thicker fibrils more quickly. We propose that differences in glycosylation (especially fucosylation) of AGP are responsible for differences in collagen fibrillogenesis and this phenomenon may contribute to the exacerbation of cartilage destruction in rheumatoid arthritis.

Renomedullary and intestinal hyaluronan content during body water excess: a study in rats and gerbils

Our previous studies in rats have suggested a role for renomedullary hyaluronan (HA) in water homeostasis. The gerbil is known for its unique ability to conserve water. In the present study renal papillary and intestinal HA were compared between groups of anaesthetized gerbils and rats before and after up to 6 h of I.V. water loading. Baseline papillary HA in gerbils was only 37 % of that in the rat. Water loading in rats increased the papillary HA content. Elevation was maximal (+27 %, P < 0.05) after 2 h of water loading and then declined to control levels after 6 h of water loading (+3 %, n.s.). In contrast, the gerbil responded with a decreased papillary HA content during water loading. The depression was maximal after 2 h (-49 %, P < 0.05) and was still 41 % below the control values after 6 h (P < 0.05). The urine flow rate increased rapidly in the rat and its maximum, 21 times above the control level (P < 0.05), occurred at the HA peak, i.e. after 2 h of water loading while in the gerbil, the urine flow rate increased slowly and slightly and was only six times above control values after 6 h of water loading (P < 0.05). The HA content along the intestine was similar in the two species: lowest in the duodenum and jejunum and highest in the distal colon. To conclude, in the rat, the elevation of papillary interstitial HA during acute water loading would counteract water reabsorption by changing the physico-chemical characteristics of the interstitial matrix favouring rapid water diuresis. This would work as a complement to the powerful regulation by ADH. The gerbil has a diametrically different regulation of papillary HA turnover during water loading. The decreased papillary HA level during water loading and the slow and small diuretic response may represent a genetic difference in adaptation to enhance the ability to conserve water in an arid environment.

Hyaluronidases and CD44 undergo differential modulation during chondrogenesis

Hyaluronan, a high-molecular-weight glycosaminoglycan of cartilage, is deposited directly into the extracellular space by hyaluronan synthases, while hyaluronan catabolism is mediated by the hyaluronidases. An in vitro cell culture system has been established in which human dermal fibroblasts are induced to undergo chondrogenesis. Here, we describe the differential modulation of the hyaluronidases and the up-regulation of the hyaluronan receptor, CD44, during such chondrogenesis. Dermal fibroblasts, plated in micromass cultures in the presence of lactic acid and staurosporine for 24 h, were then placed in serum-free, chemically defined medium. At 3 days, RNA was extracted and RT-PCR performed using primers for the hyaluronidase genes. Marked increase in HYAL1 expression was observed, with only moderate increases occurring in HYAL2 and HYAL3. No expression of HYAL4 and PH-20, the sperm-associated hyaluronidase, was detected. RNA levels correlated well with changes in hyaluronidase enzyme activity. Finally, greater expression and staining for the hyaluronan receptor, CD44s, the standard form, were detected. Differential expression of the somatic hyaluronidases and CD44-mediated hyaluronan turnover play a critical role in cartilage development from mesenchymal precursors.

Initial stages of cell-matrix adhesion can be mediated and modulated by cell-surface hyaluronan

A conceptual temporal and spatial gap exists between the first encounter of a cell with an adhesive substrate and the advanced stages of focal adhesion formation. Although ample information is available on focal adhesions structure and function, the mechanism of the first interaction events and the nature of the molecules mediating them are largely unknown. In this paper we identify cell-surface-associated hyaluronan as a mediator and modulator of the first steps of adhesion of A6 and other cells to conventional tissue culture substrates as well as to the surfaces of calcium-(R,R)-tartrate tetrahydrate crystals. Treatment of A6 cells with hyaluronidase suppresses their rapid interactions with these adhesive substrates, and incubation of either the hyaluronidase-treated cells or the substrate with hyaluronan restores cell adhesion. In contrast, excess hyaluronan on both the cells and the substrate strongly inhibits adhesion. We thus propose that cell-surface-associated hyaluronan can mediate and modulate cell-matrix adhesion at the very first encounter with the substrate. It may promote it through the establishment of exquisitely stereospecific chemical interactions or inhibit it by virtue of steric exclusion and/or electrostatic repulsion.

A requirement for the CD44 cytoplasmic domain for hyaluronan binding, pericellular matrix assembly, and receptor-mediated endocytosis in COS-7 cells

CD44-negative COS-7 cells were transfected with expression constructs for CD44H (the predominant CD44 isoform), CD44E (epithelial isoform), or truncation mutant derivatives lacking the carboxyl-terminal 67 amino acids of the cytoplasmic domain, CD44HDelta67 and CD44EDelta67. The truncation mutant CD44HDelta67 is identical to a naturally occurring alternatively spliced "short tail" CD44 isoform (CD44st), which incorporates exon 19 in place of exon 20. CD44st lacks intracellular signaling motifs as well as protein domains necessary for interaction with cytoskeletal components. Transfection of COS-7 cells with each construct yielded equivalent levels of mRNA expression, whereas no CD44 expression was observed in parental, nontransfected COS-7 cells. Western analysis and immunostaining of COS-7 transfectants confirmed CD44 protein expression of the truncation mutant derivatives. COS-7 cells transfected with CD44H or CD44E gained the capacity to bind fluorescein-conjugated HA (fl-HA) and assemble HA-dependent pericellular matrices in the presence of exogenously added HA and proteoglycan. In addition, the CD44H- and CD44E-transfected cells were able to internalize surface-bound fl-HA. COS-7 cells transfected with the vector alone or with either of the mutant CD44 isoforms, CD44HDelta67 or CD44EDelta67, did not exhibit the capacity to assemble pericellular matrices or to bind and internalize the fl-HA. Cotransfection of CD44Delta67 mutants together with CD44H reduced the size of the HA-dependent pericellular matrices. Transfection of bovine articular chondrocytes with CD44Delta67 also inhibited pericellular matrix assembly. Collectively, these results indicate an obligatory requirement for the CD44 receptor cytoplasmic domain for ligand (HA) binding, formation and retention of the pericellular matrix, as well as CD44-mediated endocytosis of HA. In addition, the results suggest a potential regulatory role for the differentially expressed alternatively spliced short tail CD44 isoform.

Expression of CD44 isoforms correlates with the metastatic potential of osteosarcoma

CD44 is a family of cell surface glycoproteins that not only function as receptors to hyaluronan and other extracellular matrix ligands but also is implicated in the invasiveness and metastasis of malignancies. The expression of CD44 and its variant isoforms was evaluated by immunohistochemistry in primary osteosarcoma, and the metastatic significance and prognostic relevance were assessed. Fifty formalin-fixed, paraffin-embedded conventional osteosarcoma samples were immunostained with monoclonal antibodies to CD44s, CD44v5, and CD44v6. When more than 50% of the tumor cells were stained, the result was regarded as ++ (overexpression). Staining patterns of CD44 mainly were membranous with a minor cytoplasmic pattern contribution. CD44s was overexpressed in 16% (eight of 50), CD44v6 in 20% (10 of 50), and CD44v5 in 46% (23 of 50) of the osteosarcomas. Of 26 patients with metastasis, CD44s was overexpressed in six (23%), CD44v6 in four (15%), and CD44v5 in 17 (65%). Overexpression of CD44v5 correlated significantly with metastasis. The survival rate of the patients in the CD44v5-positive (overexpressed) group was significantly lower than that of the patients in the CD44v5-negative group. Overexpression of CD44v5 may play a role in the metastatic behavior of osteosarcoma and may be a prognostic parameter of osteosarcoma.

Temporal expression of CD44 during embryonic chick limb development and modulation of its expression with retinoic acid

Hyaluronan-cell interactions are initiated co-ordinately with mesenchymal condensation during chondrogenic differentiation in the limb bud. Hyaluronan is responsible for the retention and organization of proteoglycan within the cartilage matrix. Hyaluronan-CD44 binding also retains proteoglycan aggregates to the chondrocyte plasma membrane. A sequence for CD44 protein in chick has recently been reported, but never evaluated in chick chondrocytes. Total RNA was isolated from embryonic chick limb buds, stages 18, 19, 24, 25 and 30. Using semi-quantitative RT-PCR, expression of aggrecan, this chick CD44 orthologue and GAPDH mRNA was analyzed. Aggrecan expression was detected at all stages, but was increased at stage 30. CD44 mRNA was detected at extremely low levels at stage 18 to higher levels in the latter stages. Thus, the temporal expression of CD44 mRNA correlated with the onset of pre-cartilage condensation. The full-length chick chondrocyte CD44 cDNA was obtained following RT-PCR using RNA derived from tibial chondrocytes from stage 37 chick embryos. The nucleotide sequence was used to generate an amino acid sequence and analyses revealed homologies of 44.4% with mouse, 47.8% with bovine and 46.3% with human CD44. Tibial chondrocytes were cultured in the presence or absence of retinoic acid for 36 or 72 h. By RT-PCR, expression of aggrecan and the CD44 mRNA by chick chondrocytes was decreased after retinoic acid treatment, while GAPDH expression showed no change. As expected, control chondrocytes exhibited a round morphology while retinoic acid-treated chondrocytes were elongated. The retinoic acid-treated chondrocytes also exhibited reduced hyaluronan binding. This functional assay indicates a role for a CD44 receptor in matrix retention by chick chondrocytes.

CD44-mediated uptake and degradation of hyaluronan

Hyaluronan turnover occurs systemically from the lymph and serum as well as locally by the same cells responsible for its synthesis. Local turnover involves receptor-mediated uptake and delivery to lysosomes. Of the many hyaluronan binding proteins/receptors known, the participation of CD44 in the internalization of hyaluronan has been best characterized. Some fraction of the hyaluronan bound to CD44 becomes internalized and delivered to lysosomes by a mechanism that is not dependent on clatherin, caveolae or pinocytosis. In cells such as chondrocytes, anabolic and catabolic cytokines can alter the activity of CD44 toward hyaluronan internalization. However, the mechanism of cellular regulation remains unclear. Regulation may involve the participation of alternatively spliced isoforms of CD44, changes in CD44 phosphorylation, changes in cytoskeletal binding proteins or, the activity or extracellular proteolytic activity.

Hyaluronan enters keratinocytes by a novel endocytic route for catabolism

Hyaluronan synthesized in the epidermis has an exceptionally short half-life, indicative of its catabolism by epidermal keratinocytes. An intracellular pool of endogenously synthesized hyaluronan, from 1 to 20 fg/cell, inversely related to cell density, was observed in cultured rat epidermal keratinocytes. More than 80% of the intracellular hyaluronan was small (<90 kDa). Approximately 25% of newly synthesized hyaluronan was endocytosed by the keratinocytes and had a half-life of 2-3 h. A biotinylated aggrecan G(1) domain/link protein probe demonstrated hyaluronan in small vesicles of approximately 100 nm diameter close to the plasma membrane, and in large vesicles and multivesicular bodies up to 1300 nm diameter around the nucleus. Hyaluronan did not co-localize with markers of lysosomes. However, inhibition of lysosomal acidification with NH(4)Cl or chloroquine, or treating the cells with the hyaluronidase inhibitor apigenin increased intracellular hyaluronan staining, suggesting that it resided in prelysosomal endosomes. Competitive displacement of hyaluronan from surface receptors using hyaluronan decasaccharides, resulted in a rapid disappearance of this endosomal hyaluronan (t(12) approximately 5 min), indicating its transitory nature. The ultrastructure of the hyaluronan-containing vesicles, co-localization with marker proteins for different vesicle types, and application of specific uptake inhibitors demonstrated that the formation of hyaluronan-containing vesicles did not involve clathrin-coated pits or caveolae. Treatment of rat epidermal keratinocytes with the OX50 monoclonal antibody against the hyaluronan receptor CD44 increased endosomal hyaluronan. However, no CD44-hyaluronan co-localization was observed intracellularly unless endosomal trafficking was retarded by monensin, or cultivation at 20 degrees C, suggesting CD44 recycling. Rat epidermal keratinocytes thus internalize a large proportion of their newly synthesized hyaluronan into non-clathrin-coated endosomes in a receptor mediated way, and rapidly transport it to slower degradation in the endosomal/lysosomal system.

Effect of hyaluronic acid on human chondrocyte cell lines from articular cartilage

The effects of hyaluronic acid (HA) derivative on the proliferation and metabolism of human chondrocytes were examined. Cells were obtained from cartilage from metatarsal phalangeal joints of 20 adult humans (aged 22-63) and from femoral knee condyles of 10 subjects (aged 22-77). Chondrocytes isolated by collagenase/Dnase digestion were cultured with addition of different doses of HA for 4 weeks. Morphological studies demonstrated that HA enhanced the adhesion of cells to substrate; HA-treated chondrocytes proliferated better than chondrocytes cultured in HA-free medium. This study shows that HA improves in vitro substrate adhesion ability and proliferative activity of human cartilage cells and that the response to the treatment varies on an individual basis.

Mouse LYVE-1 is an endocytic receptor for hyaluronan in lymphatic endothelium

The glycosaminoglycan hyaluronan is a key substrate for cell migration in tissues during inflammation, wound healing, and neoplasia. Unlike other matrix components, hyaluronan (HA) is turned over rapidly, yet most degradation occurs not locally but within distant lymph nodes, through mechanisms that are not yet understood. While it is not clear which receptors are involved in binding and uptake of hyaluronan within the lymphatics, one likely candidate is the lymphatic endothelial hyaluronan receptor LYVE-1 recently described in our laboratory (Banerji, S., Ni, J., Wang, S., Clasper, S., Su, J., Tammi, R., Jones, M., and Jackson, D.G. (1999) J. Cell Biol. 144, 789-801). Here we present evidence that LYVE-1 is involved in the uptake of hyaluronan by lymphatic endothelial cells using a new murine LYVE-1 orthologue identified from the EST data base. We show that mouse LYVE-1 both binds and internalizes hyaluronan in transfected 293T fibroblasts in vitro and demonstrate using immunoelectron microscopy that it is distributed equally among the luminal and abluminal surfaces of lymphatic vessels in vivo. In addition, we show by means of specific antisera that expression of mouse LYVE-1 remains restricted to the lymphatics in homozygous knockout mice lacking a functional gene for CD44, the closest homologue of LYVE-1 and the only other Link superfamily HA receptor known to date. Together these results suggest a role for LYVE-1 in the transport of HA from tissue to lymph and imply that further novel hyaluronan receptors must exist that can compensate for the loss of CD44 function.

Hyaluronidase can modulate expression of CD44

CD44 is a family of transmembrane glycoproteins with multiple isoforms generated by alternative exon splicing of a single gene. CD44 and its variants are expressed on a wide variety of cells including cancer cells. The mechanisms by which splice variant exons are selected are unknown. The presence of hyaluronan in the environment of the cell appears to influence that selection process. The expression of particular splice variants of CD44 as well as the simultaneous presence of hyaluronan is important for motility, invasion, and the metastatic spread of some tumors. The influence of hyaluronidase digestion on the expression of CD44 in human cancer cell lines was examined. CD44 isoforms containing alternatively spliced exons were sensitive to hyaluronidase digestion in all lines examined, but differences between cell lines were observed. Expression of CD44s, the standard form, was resistant to digestion in two of three cell lines. A tentative model was formulated proposing that CD44 isoforms containing splice variants are unstable, requiring the continuous presence of ligand for expression. CD44s is relatively more stable, not requiring the continuous presence of hyaluronan. Additionally, a number of new CD44 variant isoforms, not previously observed, were identified.

Regulation of renal proximal tubular epithelial cell hyaluronan generation: implications for diabetic nephropathy

BACKGROUND

Proximal tubular cells (PTCs) contribute to pathological changes in the renal interstitium by the generation of cytokines and alterations in the composition of the extracellular matrix. Hyaluronan (HA) is a ubiquitous connective tissue polysaccharide that regulates cell function and tissue remodeling. In the current study, we investigated the regulation of HA generation by PTCs.

METHODS

Primary cultures of human PTCs were grown to confluence and stimulated under serum-free conditions with either interleukin-1 (IL-1) or 25 mmol/L D-glucose. Alterations in HA generation were detected by enzyme-linked immunosorbent assay, and alterations in HA synthase gene expression were examined by reverse transcription-polymerase chain reaction. Subsequently, the mechanisms of IL-1 beta and glucose-induced alterations in HA were examined utilizing HK-2 cells.

RESULTS

Stimulation of human PTCs (HPTCs) with either IL-1 beta or 25 mmol/L D-glucose led to a significant increase in the HA concentration in the culture supernatant. In contrast, stimulation of HPTCs with transforming growth factor-beta1, basic fibroblast growth factor, or platelet-derived growth factor-AB did not stimulate HA production. The addition of IL-1 beta or 25 mmol/L D-glucose also increased HA generation in HK-2 cells and was associated with the induction of HAS2 mRNA. HAS3 mRNA was constitutively expressed and was not influenced by the addition of either stimulus. HAS1 mRNA expression was not detected in either unstimulated or stimulated cells. Inhibition of gene transcription or protein synthesis abolished HA production in response to either IL-1 beta or glucose. Inhibition of nuclear factor-kappa B (NF-kappa B) activation either by sulindac or by the proteosome inhibitor (PSI) abrogated both IL-1 beta and glucose-mediated alteration in HA synthesis.

CONCLUSION

This study demonstrates, to our knowledge for the first time, that increased HA synthesis in response to either IL-1 beta or elevated 25 mmol/L D-glucose is associated with NF-kappa B-activated transcription of HAS2.

Cartilage proteoglycans

The predominant proteoglycan present in cartilage is the large chondroitin sulfate proteoglycan 'aggrecan'. Following its secretion, aggrecan self-assembles into a supramolecular structure with as many as 50 monomers bound to a filament of hyaluronan. Aggrecan serves a direct, primary role providing the osmotic resistance necessary for cartilage to resist compressive loads. Other proteoglycans expressed during chondrogenesis and in cartilage include the cell surface syndecans and glypican, the small leucine-rich proteoglycans decorin, biglycan, fibromodulin, lumican and epiphycan and the basement membrane proteoglycan, perlecan. The emerging functions of these proteoglycans in cartilage will enhance our understanding of chondrogenesis and cartilage degeneration.

Identification of the hyaluronan receptor for endocytosis (HARE)

Rat liver sinusoidal endothelial cells (LECs) express two hyaluronan (HA) receptors, of 175 and 300 kDa, responsible for the endocytic clearance of HA. We have characterized eight monoclonal antibodies (mAbs) raised against the 175-kDa HA receptor partially purified from rat LECs. These mAbs also cross-react with the 300-kDa HA receptor. The 175-kDa HA receptor is a single protein, whereas the 300-kDa species contains three subunits, alpha, beta, and gamma at 260, 230, and 97 kDa, respectively (Zhou, B., Oka, J. A., and Weigel, P. H. (1999) J. Biol. Chem. 274, 33831-33834). The 97-kDa subunit was not recognized by any of the mAbs in Western blots. Based on their cross-reactivity with these mAbs, the 175-, 230-, and 260-kDa proteins appear to be related. Two of the mAbs inhibit (125)I-HA binding and endocytosis by LECs at 37 degrees C. All of these results confirm that the mAbs recognize the bone fide LEC HA receptor. Indirect immunofluoresence shows high protein expression in liver sinusoids, the venous sinuses of the red pulp in spleen, and the medullary sinuses of lymph nodes. Because the tissue distribution for this endocytic HA receptor is not unique to liver, we propose the name HARE (HA receptor for endocytosis).

Hyaluronic acid as prognostic marker in resectable colorectal cancer

BACKGROUND

Hyaluronic acid (HA), an extracellular high molecular mass polysaccharide, is thought to be involved in the growth and progression of malignant tumours. The objective of this work was to evaluate the cytosolic HA content in resectable colorectal cancer, its possible relationship with clinicopathological parameters of tumours and its prognostic significance.

METHODS

Cytosolic HA levels were examined by radiometric assay in 120 patients with resectable colorectal cancer. The mean follow-up period was 33.4 months.

RESULTS

Cytosolic HA levels of tumours ranged widely, from 30 to 29 412 ng per mg protein. Intratumour HA levels were significantly correlated with Dukes stage (P < 0.005), and were higher in patients with advanced tumours (mean(s.e.m.) 2695(446), 2858(293) and 5274(967) ng per mg protein for stages A, B and C respectively). In addition, Cox multivariate analysis demonstrated that tumour HA levels higher than 2000 ng per mg protein predicted shorter relapse-free survival and overall survival periods (both P < 0.05).

CONCLUSION

There is a wide variability in cytosolic HA levels in colorectal carcinomas, which seems to be related to the biological heterogeneity of these tumours. In addition, high tumour cytosolic HA levels were associated with an unfavourable outcome in patients with resectable colorectal cancer. HA may provide additional information to that given by other biochemical markers currently used in colorectal cancer.

Nervous system proteoglycans as modulators of neurite outgrowth

The proteoglycans are multifunctional macromolecules composed of a core polypeptide and a variable number of glycosaminoglycan chains. The structural diversity and complexities of proteoglycan expression in the developing and adult Nervous System underlies the variety of biological functions that these molecules fulfill. Thus, in the Nervous System, proteoglycans regulate the structural organisation of the extracellular matrix, modulate growth factor activities and cellular adhesive and motility events, such as cell migration and axon outgrowth. This review summarises the evidences indicating that proteoglycans have an important role as modulators of neurite outgrowth and neuronal polarity. Special emphasis will be placed on those studies that have shown that proteoglycans of certain subtypes inhibit neurite extension either during the development and/or the regeneration of the vertebrate Central Nervous System.

Stimulation of hyaluronan metabolism by interleukin-1alpha in human articular cartilage

OBJECTIVE

To determine the effects of interleukin-1alpha (IL-1alpha) on the expression of hyaluronan synthase (HAS), CD44, and aggrecan in human articular chondrocytes, and to assess the net result of these metabolic changes on the accumulation of hyaluronan within articular cartilage.

METHODS

Normal human articular cartilage slices, as well as isolated chondrocytes, were treated with IL-1alpha. Changes in the relative expression of messenger RNA (mRNA) for HAS-2, CD44, and aggrecan were determined by competitive, quantitative reverse transcriptase-polymerase chain reaction. Hyaluronan accumulation was characterized by staining with a hyaluronan-specific binding protein and by fluorophore-assisted carbohydrate electrophoresis, while proteoglycan content was determined by alcian blue and Safranin O staining, CD44 protein expression by immunohistochemistry, and aggrecan biosynthesis by 35S-sulfate incorporation. Changes in cell-associated matrix sizes were visualized by a particle exclusion assay.

RESULTS

IL-1alpha stimulated the expression of HAS-2 and CD44 mRNA (3.5-fold and 3-fold, respectively), but inhibited the expression of aggrecan mRNA. In IL-1-treated chondrocytes, extracellular hyaluronan decreased, while intracellular accumulation of hyaluronan was enhanced. Together with the decrease in expression of aggrecan, a dramatic reduction in cell-associated matrix was observed. IL-1-treated cartilage slices displayed a prominent depletion of aggrecan as well as hyaluronan within the upper layers of the tissue. The regional loss of hyaluronan coincided with a regional up-regulation of CD44.

CONCLUSION

These data demonstrate that IL-1alpha stimulates HAS-2 at the same time as it inhibits the expression of aggrecan. Although hyaluronan biosynthesis is up-regulated, so too is the expression of CD44 and the internalization/catabolism of hyaluronan. The net result is a loss of hyaluronan in areas of the articular cartilage where increases in CD44 expression are most prominent. This depletion of hyaluronan in the upper layers of the tissue likely facilitates the prominent loss of aggrecan from the tissue.

Expression of hyaluronan synthases in articular cartilage

OBJECTIVE

To investigate the mRNA expression profiles of three mammalian hyaluronan synthases (HAS1, HAS2 and HAS3) in chondrocytes from normal (undiseased) animal cartilage and osteoarthritic human cartilage maintained in experimental culture systems and exposed to catabolic or anabolic stimuli provided by cytokines, growth factors and retinoic acid.

DESIGN

Chondrocytes isolated from normal bovine, porcine or from osteoarthritic human cartilage were cultured as monolayers or embedded in agarose. Cultures were maintained for 3-5 days in the presence or absence of catabolic stimuli (IL-1, TNF-alpha or retinoic acid) or anabolic stimuli (TGF-beta or IGF-1) followed by extraction of RNA and analysis of HAS mRNA expression by RT-PCR.

RESULTS

Whereas mRNA for HAS1 was not detected in any sample, the mRNAs for HAS2 and HAS3 were expressed in human, bovine and porcine chondrocytes. HAS2 mRNA was present in chondrocytes from all cartilages and under all culture conditions, whereas HAS3 did not show such constitutive expression. In agarose cultures of bovine and porcine chondrocytes HAS2 mRNA was present in control, IL-1 and retinoic acid treated cultures, whereas HAS3 mRNA was only detected in IL-1 stimulated cultures. Mature bovine chondrocytes cultured in monolayers expressed mRNAs for both HAS2 and HAS3 in the presence of IL-1, TNF-alpha, TGF-beta and IGF-1, however immature bovine chondrocytes in monolayer cultures displayed virtually no HAS3 mRNA expression in the presence of these cytokines and growth factors. HAS2 and HAS3 mRNAs were also expressed by bovine chondrocytes isolated from either the superficial or deep zone of articular cartilage, and by human chondrocytes cultured either in the absence or presence of IL-1 and retinoic acid.

CONCLUSIONS

Our data indicate that HAS2 and HAS3 (but not HAS1) mRNAs are expressed in several mammalian cartilages. Chondrocyte HAS2 mRNA appears to be constitutively expressed while chondrocyte HAS3 mRNA expression can be differentially regulated in an age-dependent fashion, and may be affected by local and/or systemic catabolic or anabolic stimuli provided by cytokines or growth factors.

Differential effects of interleukin-1 on hyaluronan and proteoglycan metabolism in two compartments of the matrix formed by articular chondrocytes maintained in alginate

Phenotypically stable young adult bovine articular chondrocytes suspended in beads of alginate gel were first cultured for 5 days, using daily changes of medium containing 10% fetal bovine serum and supplements. The cells in the beads were then maintained in culture for a further 3 days in the presence or absence of interleukin-1alpha at 1 ng/ml in the daily change of medium. The exposure to interleukin-1alpha caused the incorporation of (35)S-sulfate into the predominant cartilage proteoglycan, aggrecan, to decrease by approximately 60%. In addition, proteoglycans that had accumulated into the cell-associated matrix during the first 5 days of culture in the absence of interleukin-1alpha moved into the matrix further removed from the cells and from there into the medium. In contrast, the exposure to interleukin-1alpha was found to markedly promote the rate of synthesis of hyaluronan, especially during the first 24 h. Over the 3 days of culture in the presence of interleukin-1alpha, a large proportion of the newly synthesized hyaluronan molecules, as well as those that had previously become residents of the cell-associated matrix, moved out of this compartment and appeared to become permanent residents of the further removed matrix. These results demonstrate that exposure of young adult articular chondrocytes to interleukin-1alpha has profound effects on the metabolism of hyaluronan, a molecule that plays a critical role in the retention of proteoglycan molecules in the matrix. Importantly, the results suggest that exposure of chondrocytes to interleukin-1 in inflamed joints, such as occurs in rheumatoid arthritis, leads to the rapid loss of coordination of the synthesis of aggrecan and hyaluronan, two of the critical constituents of the proteoglycan aggregate. In addition, we present evidence that these interleukin-1-induced effects differentially alter the metabolism of hyaluronan in the metabolically active cell-associated matrix and the metabolically inactive matrix further removed from the chondrocytes.